Dynamical intramolecular metal-to-metal ligand exchange of phosphine and thioether ligands in derivatives PtRu₅(CO)₁₆(μ₆-C)

The complexes PtRu₅(CO)₁₅(PMe₂Ph)(μ₆-C) (2), PtRu₅(CO)₁₄(PMe₂Ph)₂ (μ₆-C) (3), PtRu₅(CO)₁₅(PMe₃)(μ₆-C) (4), PtRu₅(CO)₁₄(PMe₃)₂ (μ₆-C) (5), and PtRu₅(CO)₁₅(Me₂S)(μ₆-C) (6) were obtained from the reactions of PtRu₅(CO)₁₆(μ₆-C) (1) with the appropriate ligand. As determined by NMR spectroscopy, all the new complexes exist in solution as a mixture of isomers. Compounds 2, 3, and 6 were characterized crystallographically. In all three compounds, the six metal atoms are arranged in an octahedral geometry, with a carbido carbon atom in the center. The PMe₂Ph and Me₂S ligands are coordinated to the Pt atom in 2 and 6, respectively. In 3, the two PMe₂Ph ligands are coordinated to Ru atoms. In solution, all the new compounds undergo dynamical intramolecular isomerization by shifting the PMe₂Ph or Me₂S ligand back and forth between the Pt and Ru atoms. For compound 2, ΔH‡ = 15.1(3) kcal/mol, ΔS‡ = -7.7(9) cal/(mol·K), and ΔG₂₉₈ = 17.4(6) kcal/mol for the transformation of the major isomer to the minor isomer; for compound 4, ΔH‡ = 14.0(1) kcal/mol, ΔS‡ = -10.7(4) cal/(mol·K), and ΔG₂₉₈ = 17.2(2) kcal/mol for the transformation of the major isomer to the minor isomer; for compound 6, ΔH‡ = 18(1) kcal/mol, ΔS‡ = 21(5) cal/(mol·K) and ΔG₂₉₈ = 12(2) kcal/mol. The shifts of the Me₂S ligand in 6 are significantly more facile than the shifts for the phosphine ligand in compounds 2-5. This is attributed to a more stable ligand-bridged intermediate for the isomerizations of 6 than that for compounds 2-5. The intermediate for the isomerization of 6 involves a bridging Me₂S ligand that can use two lone pairs of electrons for coordination to the metal atoms, whereas a tertiary phosphine ligand can use only one lone pair of electrons for bridging coordination.